Enantiopure Dinaphtho[2,3-b:2,3-f]thieno[3,2-b]thiophenes: Reaching High Magnetoresistance Effect in OFETs

Abstract

Chiral molecules are known to behave as spin filters due to the chiral induced spin selectivity (CISS) effect. Chirality can be implemented in molecular semiconductors in order to study the role of the CISS effect in charge transport and to find new materials for spintronic applications. In this study, the design and synthesis of a new class of enantiopure chiral organic semiconductors based on the well-known dinaphtho[2,3-b:2,3-f]thieno[3,2-b]thiophene (DNTT) core functionalized with chiral alkyl side chains is presented. When introduced in an organic field-effect transistor (OFET) with magnetic contacts, the two enantiomers, (R)-DNTT and (S)-DNTT, show an opposite behavior with respect to the relative direction of the magnetization of the contacts, oriented by an external magnetic field. Each enantiomer displays an unexpectedly high magnetoresistance over one preferred orientation of the spin current injected from the magnetic contacts. The result is the first reported OFET in which the current can be switched on and off upon inversion of the direction of the applied external magnetic field. This work contributes to the general understanding of the CISS effect and opens new avenues for the introduction of organic materials in spintronic devices.

Keywords: chiral induced spin selectivity effect; chirality; magnetoresistance; organic semiconductors; spin; transistors.

Figure 1

Molecular structure of (R)‐DNTT (pink), (S)‐DNTT (green), C8‐DNTT, (R)‐EH‐DNTT, (S)‐EH‐DNTT, and BTAT.

Figure 2

a) Crystal packing diagram of (S)‐DNTT in the herringbone plane without the chiral alkyl chains. Molecules in pink and red are crystallographically independent one with respect to the other. The black dashed lines represent S–S short contacts, the herringbone angle is displayed in grey. b) (S)‐DNTT intermolecular transfer integrals (in meV) calculated from the experimental crystal structure. c) Front view of the Hirshfeld surfaces of the two distinct molecules belonging to the same unit cell of (S)‐DNTT with S–S contact regions highlighted in yellow.

Figure 3

a) Geometry of a BGBC device with Au electrodes, b) representation of a BC device in a parallel magnetic field, and c) in an antiparallel magnetic field. Gate contact (G) is depicted in gray, the dielectric in light blue, source (S) and drain (D) gold contacts in orange and the semiconductor in yellow. The direction of the magnetic field (B) and of the hole current (h⁺) are depicted with black arrows. d) Transfer curves of (R)‐DNTT (green) and (S)‐DNTT (pink) best performing OFETs fabricated with BGBC geometry at a substrate temperature of 40 °C. e) Transfer curves of (S)‐DNTT BC OFETs placed in a parallel (red) and antiparallel (blue) magnetic field.

Figure 4

Geometry of a BGBC device with ferromagnetic electrodes a) in a parallel and b) in an antiparallel magnetic field (B). Gate contact (G) is depicted in gray, the dielectric in light blue, source (S) and drain (D) contacts in orange (Au) and dark grey (Ni) and the semiconductor in yellow. The direction of the magnetic field (B) and of the hole current (h⁺) are depicted with black arrows. c) Transfer curves in saturation regime (V _d = −4 V) of (R)‐DNTT and d) (S)‐DNTT in a parallel (red) and antiparallel (blue) magnetic field. In the graph is reported a schematic representation of the injection of parallel (pink) or antiparallel (green) charges from the ferromagnetic contact (grey area) into the chiral semiconductor (yellow area).